1. Field of Invention
The present invention relates to a method and a mask for crystallizing semiconductor material, and more particularly to a method for crystallizing amorphous silicon into polysilicon (poly-Si) and a mask used therefor.
2. Description of Related Art
In recent years, to meet the requirements of the high-performance flat-panel display and the integrated circuits for the panel, the low-temperature poly-Si thin-film crystallization technology has been widely researched. Excimer laser crystallization has become the mainstream of current crystallization technology.
The sequential lateral solidification (SLS) crystallization technique is a modified of the excimer laser crystallization technique. The original excimer laser system is further provided with a substrate platform in a submicron movement and a high-precision optical system, and the laser beams are modulated by the mask design in the optical system, for controlling the film lateral solidification, crystallization region, and the positions of the grain boundary. Thus, a polysilicon film with a periodical grain boundary arrangement is obtained. Therefore, the grain size and the film crystallization quality obtained by the SLS crystallization technique are closely related to the modulated mask design in the optical system.
With a proper mask design, the SLS laser crystallization device can be used to prepare a low-temperature polysilicon thin-film transistor device with high carrier mobility. The size of the obtained polysilicon grain can be larger than that of the grain obtained by the conventional excimer laser crystallization technique, and has a preferred uniformity. Therefore, the SLS crystallization technique is regarded as a promising crystallization technique for the next generation. However, although a polysilicon film with larger grains can be obtained through the SLS crystallization technique, high surface roughness and thin-film protrusion with defects are still observed in the SLS crystallized poly-Si thin-films.
FIGS. 1A to 1D are schematic views for an actual grain growth. A mask is disposed above the substrate 10, and then laser irradiation is conducted, such that the regions irradiated by the laser in the amorphous silicon film on the substrate 10 are melted into a liquid phase silicon film 11. Then, as shown FIG. 1A, the solid amorphous silicon film without being irradiated by the laser induces the liquid phase silicon film 11 to laterally solidify into polysilicon 14, as shown in FIGS. 1B to 1D, until the poly-Si grains at the two sides touch each other, as shown in FIG. 1E. Through the above crystallization method, large-sized grains with periodical arrangements can be fabricated. However, as shown in FIG. 1E, an obvious film protrusion 16 occurs at the region where the grains touch each other, after lateral grain growth. These film protrusions cause device electrical characteristics and reliability degradations.